GLYCOGEN: ROLE AND FUNCTION
When you do physical activity, you need energy. More specifically, your muscles need energy .
To prepare for exertion, they build up a reserve from which they can draw quickly, at any time: glycogen .
Optimizing these reserves therefore has a direct impact on your performance, and that's why it's important to understand how glycogen works.
DEFINITION
Simply put, glycogen is a form of glucose (sugar) stored in your muscles and liver .
This sugar serves as a reserve which will then be used in different contexts.
It's a selfish reserve. It only serves the muscle in which it's stored. If you run, your legs use their reserve, but can't borrow from your arms, for example.
This is the generous reserve. It mainly serves to nourish your brain and vital organs by maintaining a blood sugar level (glycemia).
If your muscle reserves are depleted, it can also compensate for this lack.
Imagine a car.
- Glucose is the fuel that makes the engine run.
- Glycogen is like a fuel tank. You refill it regularly to be able to drive, and you use fuel when you're moving. Once the tank is empty, you have to refill it.
HOW DOES IT WORK?
During physical exertion, your body uses two sources of fuel: your glycogen and your fats .
Each of them will be used in a different context:
The more intense the effort (sprint, endurance, HIIT, ...) the more the muscles demand glucose because it burns quickly and is therefore more easily usable than fat.
It is mainly used for low-intensity activities (walking, stretching, etc.). In this context, it has time to convert fat into energy using oxygen.
When the intensity is moderate, the body will tend to draw from both sources.
Limited resource
Unlike fat (of which you have almost unlimited reserves, even if you are very lean), glucose stored as glycogen is very limited : it lasts about 1h30-2h during sustained effort.
That's why marathon runners take gels : they add glucose to the blood to prevent their muscle glycogen stores from being completely depleted.
If you run out of your supply and don't have a way to replenish it quickly, the consequence is that your body will end up breaking down proteins from your muscles to create glucose.
This is called gluconeogenesis.
You may therefore lose muscle mass if you exhaust yourself too much, without sufficient intake to compensate (one of the reasons why marathon runners tend to be less muscular).
Namely
Your body constantly adapts to the stress it faces. The more you train, the more glycogen you will be able to store in your muscles.
GLYCOGEN RECHARGE
To avoid running out of glycogen, there are several things to keep in mind if you want to optimize refueling.
As mentioned, the body can produce glucose via gluconeogenesis .
For example, it recycles lactate (which you produce during exercise), amino acids, or your fats to produce new energy. However, this is a slow and energy-intensive process that takes between 24 and 48 hours (depending on the intensity of the session) .
Right after exercise, your insulin sensitivity is at its maximum : this is the moment when your body is most efficient at converting sugar into glycogen rather than fat.
Consuming carbohydrates at this time allows you to quickly replenish your reserves.
You can consume high glycemic index (GI) carbohydrates right after training to accelerate your glycogen replenishment (white rice, ripe bananas, honey, ...).
The rest of the time , aim for low GI foods (sweet potato, quinoa, legumes, ...).
If you are doing several sessions in the same day or are in a competition phase, consuming 1g/kg of body weight/h, for 4 hours following exercise , will allow you to load your reserves ( source ).
However, this represents a huge amount of carbohydrates and, most of the time, having a normal intake will be enough to replenish your reserves within 24 hours.
It is quite well known that combining protein and carbohydrate facilitates glycogen replenishment .
When you mix the two, you get a stronger and faster insulin response than if you ate the carbohydrates alone, which makes it easier for glucose to enter your muscles.
Learn more about insulin and how it works
Anabolic window
The anabolic window is a debated concept in nutrition.
The idea is that during and immediately after exertion, your body undergoes a physiological change:
- Insulin sensitivity increases , so your muscles absorb nutrients (and therefore glucose) much better.
- Protein synthesis increases to repair the damage caused by your workout.
For a long time, it was believed that this opportunity lasted only 30 to 45 minutes after exertion, however the data has changed:
The rate of resynthesis is highest in the first 2 hours , then it decreases but remains high for nearly 24 to 48 hours .
Muscle building remains enhanced for at least 24 hours after a weight training session.
A recent meta-analysis even highlights that the timing of protein or carbohydrate intake is irrelevant, as long as one respects their daily intake.
IN SUMMARY
Glycogen is a reserve of glucose stored in your muscles and liver, used as a quick fuel when your body needs it.
During exercise, the higher the intensity, the more your body draws on these reserves before resorting to fats.
These reserves are limited , hence the importance of understanding how it all works in order to best optimize your performance and recovery.
FAQ
Yes.
Reduced insulin sensitivity makes glucose less efficient at entering the muscles. Glycogen replenishment is therefore slower , which can prolong fatigue, reduce the quality of recovery, and limit performance in closely spaced training sessions.
This does not prevent recovery, but it becomes less optimal with the same intake.
Yes.
The body can produce glucose via gluconeogenesis (from proteins, lactate, or glycerol), but this process is slow and energetically costly .
It is sufficient if the time between sessions is long, but becomes limiting if the efforts are close together or intense.
Yes, definitely .
Chronic stress and lack of sleep disrupt glucose management and slow glycogen replenishment. With the same food intake, recovery is less effective .
Yes.
It plays a role in regulating blood sugar, brain function, stress tolerance, and even the perception of fatigue. It's not just a matter of performance, but of overall energy metabolism.
Because glycogen stores drop rapidly. The body has to learn to reorganize its energy priorities, which takes time.
This transitional phase explains the initial drop in energy.